How Tight Should Spark Plugs Be? What If They’re Too Loose or Tight?
In this guide, we detail how tight you need to set your spark plugs and how to do so. If they are too loose or too tight it can lead to poor engine performance or even damage, so you want to make sure you get this right!
Last Updated: April 15, 2023
In many respects, car maintenance involves a balancing act. Wait too long between tune-ups and your car might suffer for it. But being over-zealous can lead to avoidable costs.
Your vehicle is a complex piece of engineering. When installing or replacing components, if something doesn’t fit the way it should, it can affect how your vehicle performs. This is never more true than with spark plugs.
In this article, we answer the question: how tight should spark plugs be? We’ll also discuss the symptoms associated with over and under-tightened spark plugs and teach you how to install your spark plugs just right the first time of asking.
Contents (Jump to Topic)
- 1 Key Takeaways
- 2 How Tight Do Spark Plugs Need to Be?
- 3 What Will Happen if Spark Plugs Are Too Loose?
- 3.1 Poor Engine Performance
- 3.2 Engine Pre-Ignition
- 3.3 Harmful Detonation
- 3.4 Dead Cylinder
- 4.1 Hard or Impossible to Remove
- 4.2 Damaged Cylinder Head
- 4.3 Foreign Objects in Combustion Chamber
- 5.1 During Installation
- 5.2 After Installation
- 6.1 Equipment Needed:
- 6.2 Step 1: Allow Engine Time to Cool Down, Disconnect Battery & Wipe Down Loose Debris
- 6.3 Step 2: Removal: Use Penetrating Oil If Stuck
- 6.4 Step 3: Clean Cylinder Heads & Spark Plug Thread
- 6.5 Step 4: Ensure Replacements Aren’t Damaged & Gap New Plugs
- 6.6 Step 5: Use Your Fingers to Screw the Plug In & Tighten
- 6.7 Step 6: Reattach Wire and Boot
How Tight Do Spark Plugs Need to Be?
Without spark plugs, your car could not run. They provide the spark to ignite the fuel-air mixture, the driving force of power in an internal combustion engine.
Unfortunately, there’s no such thing as universal plugs. The diameter and build can vary based on their intended usage. For instance, units for an eight-cylinder engine won’t be the same as ones for a four-cylinder.
If you’re unsure what size your spark plugs should be, refer to the user manual. Once you know the correct size needed, follow our correct installation practices below. This way, no matter what car you have, you will end up with an optimal fit.
What Will Happen if Spark Plugs Are Too Loose?
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Thanks to your engine mount, you shouldn’t hear or feel your motor much from the cabin. Older vehicles may be equipped with rubber mounts, whereas newer ones have hydro-elastic varieties.
Despite this, it’s impossible to repress vibrations internally. Improperly fitted plugs can grow looser the longer you drive around with them.
Naturally, this will influence how affected cylinders perform. Imagine plugging an electrical device in only halfway – it won’t always work to the best of its capabilities, or sometimes even at all.
If your plugs aren’t tightened correctly and are too loose, your engine is highly likely to experience one of the following:
- Problems with engine performance.
- Dead cylinder.
Poor Engine Performance
If a plug isn’t fully screwed into the cylinder head, it can lead to:
- Trouble getting the engine to start.
- Slow, difficult acceleration.
A loose plug can overheat significantly, which can induce abnormal combustion. One common side effect of this is pre-ignition.
The fuel-air mixture lights due to an overheated surface, such as that of the spark plug. Normally, this should only occur when it sparks.
Pre-ignition can encourage detonation. This is spontaneous combustion inside the engine that can manifest as pinging or knocking. Engine damage or total failure are typical results of this detonation.
If the poorly-installed plug is too loose, it could pop out. A dead cylinder, while the vehicle is in motion, can be a shock to you and your motor.
What Will Happen if Spark Plugs Are Too Tight?
Too tight spark plugs can damage the cylinder head, and make them impossible to remove.
Remember that balance we talked about earlier? Plugs that are too snug can have unpleasant repercussions too, including:
- Stuck units that won’t come out.
- Damaged cylinder head.
- Pieces of the broken plug falling into the combustion chamber.
Hard or Impossible to Remove
At the least, removing an over-tightened spark plug will be difficult. Worst case scenario, the plug won’t come out at all.
It may even appear as though the spark plug has melded with the head of the cylinder. Deposits, debris, and potential corrosion build-up can behave like glue.
Damaged Cylinder Head
The bolt hole inside the cylinder head needs to stay intact. If it’s stripped or otherwise deformed by using too much torque, a replacement plug can’t be installed.
Removal of a jammed spark plug can cause damage to the bolt hole. If the problem is severe, repair costs can be expensive.
Foreign Objects in Combustion Chamber
Broken pieces of the spark plug insulator or electrode can find their way into the combustion chamber. They can knock around, harming the parts and surfaces they hit.
How to Know Spark Plugs Aren’t Too Loose or Too Tight?
The surest way is to inspect and handle the plug directly, as discussed in depth in our guide to spark plug diagnosis. If they’re too loose, you may be able to turn the plug with your fingers. Too tight, and it won’t budge at all, even with a spark plug or torque wrench.
There are several indicators that your plug installation may not have gone as smoothly as you thought. If any of these apply to you, pop the hood for a check.
Did you adhere to the manufacturer’s guidelines for fitting while installing your spark plug?
Certain brands have separate specifications for new versus refitted units. For example, a new plug may require a third of a rotation with a spark plug wrench. A plug that you’ve cleaned and want to refit may require one-and-a-third rotations.
If you were using a torque wrench, were you aware of the suggested torque specs? Or did you play it by ear?
Hearing, feeling, or seeing breaks in the plug should never be dismissed. They’re signs that you may have gone overboard, fracturing the plug.
Erratic motor performance after a replacement could warn of poor plug installation. Take note of the symptoms we talked about earlier.
How to Tighten Spark Plugs
Don’t be intimidated if you got it wrong the first time.
First of all, check the following video for a visual guide for a successful replacement, then read the guidelines below.
- New units (if old ones are faulty, broken, or aged).
- Appropriately-sized spark plug wrench (preferable) or torque wrench.
- Clean rags.
- Penetrating oil (for trapped devices).
- Thread chaser or cylinder-head cleaning kit.
- Feeler gauge.
Our guidelines will help you avoid both extremes of loose and tight, for an optimal installation:
- Allow engine time to cool down completely.
- Disconnect battery.
- Wipe down loose debris.
- Removal: use penetrating oil if the unit is stuck.
- Clean cylinder heads.
- Ensure replacements aren’t damaged.
- Gap new plugs.
- Use your finger to screw it in.
- Use a wrench to tighten, being careful.
- Reattach wire and boot.
Step 1: Allow Engine Time to Cool Down, Disconnect Battery & Wipe Down Loose Debris
Give your motor time to cool off before you open the hood. Put your gloves on so that you don’t forget.
No one wants to get shocked while working. To keep yourself safe, disconnect the negative terminal, and set the connector aside.
Use your rags to wipe on and around the plugs. You don’t want any deposits or debris falling into the cylinder head during removal.
Step 2: Removal: Use Penetrating Oil If Stuck
Detach the spark plug boot and wire attached to the unit. If your wires aren’t labeled, either label them or work on them one at a time to avoid confusion.
Use your wrench of choice to unscrew the plug. If it isn’t coming out, apply penetrating oil and wait a while. This will dissolve the crud that’s holding the plug to the bolt hole.
Try again. If it’s still not coming loose, repeat an application of oil. Shift the unit backward with your fingers, re-tightening it, to distribute the oil.
You may have to do this back and forth several times. The goal is to loosen the plug without forcing it or breaking it.
Step 3: Clean Cylinder Heads & Spark Plug Thread
Using the thread chaser tool or cleaning kit will get rid of troublesome debris still caught in the bolt hole or spark plug threads.
Step 4: Ensure Replacements Aren’t Damaged & Gap New Plugs
It’s admittedly rare, but defects can happen. If you notice cracks or chips, dispose of the new plug.
Even if the manufacturer states they’re pre-gapped, double-check using your feeler gauge. Adjust as needed.
Step 5: Use Your Fingers to Screw the Plug In & Tighten
Position the plug and screw it in. If you come up against resistance, it may be at an angle. If so, reposition and try again.
Once it’s in, tighten the spark plug using the appropriate wrench to secure it. Stick to the manufacturer guidelines for proper torque setting here; don’t go overboard.
Step 6: Reattach Wire and Boot
Once the installation is complete, reattach the spark plug wire and boot. Repeat the procedure with all the units that need to be replaced.
Going to a Professional to Tighten Spark Plugs: What to Expect
You’re not obliged to do this by yourself. You can go to your local mechanic and have them do the replacement for you.
This will cost you more, though. Expect to pay between $40–$150 for professional installation, excluding new units.
How Tight Should Spark Plugs Be? Wrapping It Up
The ideal tightness of spark plugs will depend on their specific type and the make and model of your vehicle.
Most importantly, be precise with the installation. Don’t rush, and be sure to use common sense. You don’t need herculean strength to tighten them; a light touch is enough.
Do you have questions for us or tips on spark plug installation? We welcome your feedback, so please share your thoughts and queries in the comments below and we’ll be sure to respond!